The present invention relates to an electro-slag refining (ESR) or remelting process and more particularly, a method for utilizing and deoxidizing the slag to control contamination of and alloy variations in the remelted product of such process. Such method was developed principally for remelting and refining high strength alloy steels designated for critical high temperature applications, such as turbine rotor shafts, etc. Experience had shown that high residual aluminum in the alloy steels, in excess of 0.008 to 0.010%, by weight, caused such steels to fail by creep rupture. Thus, a primary aim of the research leading to the development of this invention was to devise a slag system for an ESR process that would control aluminum contamination, while avoiding other pitfalls, such as hydrogen and oxygen pickup, and alloy fade or variations in the remelted alloy steel ingot.
Though ESR has been known and practiced for years, the sophisticated nature of the metals involved, and the critical applications therefore, called for special considerations.
According to the description by Duckworth and Hoyle in Electro-slag Refining, published in 1969 by the British Iron & Steel Research Association, ESR is a secondary refining process for metal, using as its raw materials a solid consumable electrode of such metal in the form of a wrought or cast ingot, or scrap. The ESR process uses a molten slag bath for melting said electrode. The slag bath, contained in a cooled mold, is resistance heated, melted, and maintained in a molten condition by an electric current flowing between said electrode and a cooled base. As the temperature of the slag bath rises above the melting point of the electrode metal, droplets melt off the tip of the submerged electrode, fall through the slag bath, and collect in a pool on the base to solidify. The electrode is continuously fed into the slag bath, and an ingot, the remelted product of said electrode metal, which now acts as the secondary electrode is progressively built up. With such buildup, the molten slag is continuously displaced in an upward direction.
Since development of the ESR process prior to WWII by Robert Hopkins, activity therein has remained low key even though the interest has been quite keen. As a result there is considerable published literature and world-wide patents directed to ESR and to improvements thereof.
By way of example, U.S. Pat. No. 4,061,493, to Jaeger, relates to a process to improve the purity of the remelted product (ingot) of an ESR process. This is achieved in the ESR process by the steps which include melting at least one self consuming electrode with alternating current in a liquefied electrically conductive slag. Concurrently, superimposed currents are generated in the ESR slag by means of at least two differently poled non-melting auxiliary electrodes connected to at least one d.c. source (1) between the auxiliary electrodes and the remelting electrode, and (2) the auxiliary electrodes and the ingot. This results, by utilization of fusion electrolysis, in a migration of the undesirable elements, i.e. H.sub.2 and O.sub.2, present in the form of ions, to the auxiliary electrodes and the removal thereof from the melt.
Another aspect of ESR which has received interest is the field of fluxes, the slag forming ingredients. U.S. Pat. No. 3,950,163, to Nafziger, teaches the use of a quaternary flux for ESR to lower the liquidus temperature of the slag while maintaining its electrical resistivity. Such a flux comprises CaF.sub.2, CaO, MgO and Al.sub.2 O.sub.3.
U.S. Pat. No. 3,857,702, to Corbett, teaches an ESR flux formed from particulate batch materials providing at least alumina, a fluoride and alkaline earth metal oxide, including calcium oxide, and the process to minimize the presence of free CaO, by which such flux is made. An exemplary flux taught by Corbett comprises 40% CaF.sub.2, 30% CaO, and 30% Al.sub.2 O.sub.3, with free calcium oxide being less than 1.5%.
Typically among such prior art ESR practices, and particularly the fluxes used therein, a common fact appears--the use of a single flux, from start-up through melting. In contrast to this, the preferred embodiment of the present invention employs an essentially two flux system with slag deoxidation. The timing and/or manner in which the two fluxes are utilized in the preferred practice of this invention, and the manner in which the slag is deoxidized will be described in greater detail hereinafter.